Influence of Counter-ions on Antifogging Coatings
by
Shashank Sundareshan
SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND
ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE
DEGREE OF
MSSACHUSETTS
INSTFTUTE E
OF TEGHNOLOGY
BACHELOR OF SCIENCE
AT THE
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
JUN 182008
LIBRARIES
MAY 2008
ARCHIVES
The author hereby grants to MIT permission to reproduce and to publicly distribute paper
and electronic copies of this thesis document in whole or in part in any medium now
known or hereafter created.
7/
/,
Signature of Author
-
Department of Materials Science and Engineering
May 9, 2008
Certified by
,
Michael F. Rubner
Professor of Materials Science and Engineering
Thesis Supervisor
Accepted by
Caroline A. Ross
Professor of Materials Science and Engineering
Chair, Departmental Undergraduate Committee
Influence of Counter-ions on Antifogging Coatings
by
Shashank Sundareshan
Submitted to the Department of Materials Science and Engineering on May 9, 2008 in
Partial Fulfillment of the Requirements for the degree of Bachelor of Science in Materials
Science and Engineering.
Abstract
The influence of different counter-ions on the superhydrophilic and antifogging behavior
of polyelectrolyte multilayers was examined. Multilayers assembled with a polymer
anion and amine-modified silica nanoparticles were treated with salt solutions of
monovalent and divalent cations. Refraction index of the films dropped significantly
when treated with monovalent cations, and increased when treated with divalent cations.
It has been found that the refraction index decrease in films treated with monovalent salts
may be correlated with porosity increase in treated films, and this porosity increase in
turn linked with an increase in hydrophilicity. Polyelectrolyte films have yet to show
long-lasting antifogging properties on the commercially valuable polycarbonate, but
monovalent cations may have improved the longevity of antifogging properties of the
multilayer on polycarbonate, and warrant further study.
TABLE OF CONTENTS
................................. 2
ABSTRACT ................................................................................
TABLE OF CONTENTS ..................................................................... ................................. 3
LIST OF FIGURES AND TABLES ............................................................ .......................... 4
ACKNOWLEDGEMENTS................................................................................................
5
6
Chapter 1 Introduction................................................. ..............................................
1.1 Introductory Remarks ................................................................. .............................. 6
0
1.2 Project Goals.................................................................................................................
...........
10
1.2.1 Thesis Objectives .................................................................. ...
1.2.2 Thesis Motivation ............................................................... ............................ 10
............................... 11
1.2.3 Current Research Advancements.........................................
Chapter 2 Design Parameters.......................................... .................................................. 12
2.1 Introduction to Design Idea ............................................................ ......................... 12
12
2.2 Exploratory Experimentation ......................................................
2.2.1 Thickness Consideration and Calibration ..............................................
12
12
2.2.2 Free Carboxylic acid Group Content ..........................................................
2.2.3 Film Stability Under pH Stress....................................................................
13
14
2.3 Exploratory Experimental Methods ...................................................................
14
2.3.1 Polyelectrolyte Films on Glass Slides............................... .............
........ ......... 14
2.3.2 Film Stability Under pH Stress..............................................
15
2.3.3 Free Carboxylic acid Group Content ..........................................................
15
2.4 Exploratory Experimental Results .....................................................................
19
2.5 Experimental Setup Conclusions ........................................................................
19
........
Chapter 3 Counter-ion Influence on Antifogging PEMs....................................
3.1 Polyelectrolyte Films on Polycarbonate Slides .......................................
........
19
3.2 Counter-ion Substitution ...................................................................... ................... 20
3.3 Film Characterization.................................................... .......................................... 20
3.3.1 Anti-Fog Test...................................................... ............................................ 20
........ 21
3.3.2 Contact Angle Measurement .................................... .........
3.3.3 Durability Test............................................. ............................................ 21
......... ......... 21
3.3.4 Humidity Chamber Aging .....................................................
3.3.5 Ellipsometry Measurements ..........................................................................
21
3.4 Results .....................................................................................
............................... 22
3.5 Discussion ..................................................................................
............................ 28
Chapter 4 Summary and Future W ork ....................................................... 30
4.1 Thesis Summary ............................................................................. ........................ 30
4.2 Future Directions ............................................................................ ........................ 30
4.3 Bibliography ................................................................................................................. 31
Appendix A .......................................................................................
................................. 32
LIST OF FIGURES AND TABLES
Figure 1. LbL growth process for a negatively charged substrate ................................................ 6
Figure 2. Polyectrolyte charge dependence on pH .........................................
...............
Figure 3. APSiO2/PAA film on negatively charged substrate .................................................. 8
Figure 4. Introduction of cations to APSiO 2/PAA film (include x-linking)................................ 11
Figure 5. Methylene blue molecular structure ..........................................
............ 13
Figure 6. Calibration curves for films grown at a) pH 3.0, b) pH 2.5, and c) pH 2.0...........16-17
17-18
Table 1. Results of pH dips for different growth systems .....................................
18
Figure 7. Degradation of pH treated films.....................................................................
Table 2. Methylene blue stained absorbance results of post-assembly treated films........... 18-19
Figure 8. Dynamic contact angles of counter-ion dipped films.................................. 23-24
Figure 9. Final contact angles of counter-ion dipped films......................................25
Table 3. Development of anti-fogging properties over humidity aging time .................
26
Figure 10. Anti-fog test of unaltered, Na-modified, and Li-modified films ............................ 26
Table 4. Mechanical Durability of counter-ion films under rubbing test................................27
Table 5. Film thickness and refractive index .................................................. 28
Table 6. Film thickness and refractive index without roughness layer .................................... 28
Figure 11. Anti-fog tests on unaltered films before and during humidity aging ...................... 32
Figure 12. Anti-fog tests on Na+-modified films before and during humidity aging ............... 33
Figure 13. Anti-fog tests on Li+-modified films before and during humidity aging ................ 34
Figure 14. Anti-fog tests on K+-modified films before and during humidity aging ................. 35
Figure 15. Anti-fog tests on Mg 2+-modified films before and during humidity aging ................ 36
Figure 16. Anti-fog tests on Ca2+-modified films before and during humidity aging .............. 37
Figure 17. Anti-fog tests on Ba2+-modified films before and during humidity aging .............. 38
ACKNOWLEDGEMENTS
I would like to thank my graduate student advisor, Zekeriyya Gemici, for all of
his help in teaching me a variety of experimental techniques and data analysis and in
guiding me to a better understanding of the project. Additionally, I would like to thank
Jenny Lichter for training me in the methylene blue staining technique and analysis, as
well as Tim McClure for training me to use a profilometer and spectrophotometer. This
help proved invaluable in deciding which experimental setup to use. I would also like to
thank Dr. Koushik Mukherjee and Hiroomi Shimomura for providing me materials with
which to complete my polymer films. I especially want to thank Professor Michael F.
Rubner for allowing me the opportunity to participate in such an exciting field and on
such a meaningful and directly applicable topic. Lastly, I would like to thank the
Department of Materials Science and Engineering, the Center for Materials Science and
Engineering, and the Institute for Soldier Nanotechnology for allowing me access to
state-of-the art characterization tools and facilities.
Chapter 1: Introduction
1.1 Introductory remarks
Polyelectrolyte multilayers (PEMs) are films grown by layering alternately charged
polyelectrolytes, or polymers with charged groups. Alternately, PEMs can be grown with
a combination of polyelectrolytes and charged nanoparticles. Because of the great deal of
engineering control available in the surface modification of these multilayer films, they
have been heavily researched for a variety of novel uses including creating antibacterial,
antifogging, and antireflective coatings, as well as uses in fuel cells. Properties such as
thickness, composition, and porosity, can greatly alter the characteristics of the film.
PEMs are typically grown using the Layer-by-Layer (LbL) method, shown in Figure 1.
L
I
UT
Polycation
UT
Rinse
Polvanion
Rinse
Figure 1. LbL growth process for a negatively charged substrate. If the substrate were to
be positively charged, the polycation and polyanion in the figure would be switched.
The LbL method consists of dipping a charged substrate, such as a negatively charged
soda-lime glass slide, into an oppositely charged polyelectrolyte or nanoparticle solution.
The slide is then rinsed in water or other solutions to remove excess solute on the slide,
and the slide is left with a single layer of charged polymer or nanoparticles. This slide is
then dipped in a solution containing an oppositely charged material and then rinsed again.
This creates the first bilayer. This process can be repeated for as many bilayers as
necessary.
The pH at which PEMs are assembled determines their stability. Using poly(acrylic acid)
(PAA) and 3-aminopropyl modified silica nanoparticles(APSiO 2) as a model, the
difference in most common electrostatic charge at pH 3 and pH 7 is shown in Figure 2.
This is due to the pKa value of the carboxylic acidic acid groups of PAA and the amine
groups of APSiO 2. A pKa value is the pH at which a molecule or material has a fifty
percent chance of being protonated. Below that pH, the molecule has a higher probability
of being protonated, while above the pKa, the molecule has a lower chance of being
protonated. As the pH is increased, the charge density on PAA increases, while the
charge density on APSiO 2 decreases. Stable films can be grown between pH 2 and pH 4.
However film stability is largely pH dependent. Furthermore, charge density dependence
on pH is important in this experiment because counter-ions were added at pH 7 to interact
with carboxylic acidic groups. Figure 3 shows a single bilayer of PAA/APSiO 2 on a
negatively charged substrate. The positively charged APSiO 2 binds to both the negatively
charged PAA and substrate.
-CH,-
CH
-
-CH, - CH-)
C--O
H+
O-
PAA at pH 3
C
O
O
H
PAA at pH 7
NH 3
NH,
NH 3
NH,
APSiO, at pH 3
APSiO, at pH 7
Figure 2. Polyectrolyte charge dependence on pH. Molecules shown are the most likely
state of that molecule (either protonated, neutral, or deprotonated) at the pH indicated.
CO"-
?
NH
+
NH
+
--------
? +
NH 3
? +
NH 3
?+
NH
3
Substrate--------
Figure 3. APSiO2/PAA film on negatively charged substrate (one bilayer).
PEMs have been grown and modified to create and increase antifogging properties of the
films, and research groups have been able to produce various PEMs that have
demonstrated robust anti-fogging. l Fog is produced when fluid condenses on a film in
discrete droplets that can scatter light. When the contact angle, defined by the angle
satisfying Young's equation made by the fluid and the surface, is low, light scattering
events do not occur and the film will transmit light. However, when the contact angles
are high, droplets form and scatter light effectively, creating a foggy appearance. 2 A high
energy surface that is roughened will be able to form a continuous or near-continuous
film of liquid on the surface, diminishing the amount of fog created. 3
This has been done by creating superhydrophilic surfaces that exhibit nanoporosity. The
superhydrophilicity of the film creates the anti-fogging property by inhibiting the
formation of water droplets that would scatter light. The nanoporosity of the film ensures
that the water can wick into the multilayer system rather than simply spreading on the
surface. It is this wicking that gives the film the superwetting characteristic, producing
water contact angles of less than 5 degrees on the film. Films of poly(allylamine
hydrochloride) (PAH) and SiO 2 nanoparticles have been investigated by Cebici et. al. that
support the theory that nanoporosity in a hydrophilic PEM can greatly aid wetting
properties. Findings suggest that superwetting occurs when water fills the surface
microstructure.' The Rubner group has further investigated the PAH/ SiO 2 films, using
the SiO 2 nanoparticles of different size to create the desired nanoporosity in films.
Unpublished results find that the superwetting properties are created by capillary
imbibition of water droplets into the nanoporous structure of the film. Instead of simply
filling in the nanopores, however, the water builds up as shells around nanoparticles.
Capillary condensation, the process by which fluid is moved and confined to nanopores
due to the lower pressure created in those nanopores, is the cause of the wicking that
inhibits water droplets from forming. Good anti-fogging properties have been observed
with the PAH/ Si0 2 films.
However, these films do not retain their antifogging properties after aging in a humidity
chamber. The Rubner group has also studied another type of film that does retain its
antifogging properties even after humidity aging for 3 days. PAA/ APSiO 2 films grown
on glass have been shown to have more durable antifogging properties. The difference
between the PAH/ SiO 2 films and the PAA/ APSiO2 films is that in the former, the
polymer contains the positive charge, whereas the polymer contains the negative charge
in the latter. This reversal on soda lime glass produces a denser packing for the PAA/
APSiO 2 films, which leads to lesser porosity but surprisingly greater antifogging
longevity.
However, there is an ongoing search for a long-lasting durable film that will retain its
antifogging properties after a long term humidity test on all commercially relevant
substrates. While APSiO 2/PAA films perform well in a 3-day humidity aging test on
glass, they are not yet as long-lived on polycarbonate and quartz.
1.2 Project Goals
1.2.1
Thesis Objectives
The objective of this thesis is to observe the effect of titrating carboxylic acidic acids in
PEM structures with various counter-ions (e.g. Na+, Li+, K%, Mg2+, Ca 2+, and Ba2+).
Specifically, titrating PEM structures with highly hydrophilic counter-ions should impart
or improve the anti-fogging effects or film wettability. Another objective of this thesis is
to observe the effect of various counter-ions on the mechanical durability of the film.
1.2.2
Thesis Motivation
Antifogging coatings will have an enormous commercial and safety impact on today's
world. Improved antifogging coatings would be used on windshields, eyeglasses, safety
glasses, windows, and ski and SCUBA goggles. Furthermore, antifogging coatings would
help reduce inefficiencies in greenhouse windows and solar arrays.2 Specifically, this
thesis focuses on retaining antifogging properties after humidity aging. Antifogging films
that have a long antifogging property lifetime in high humidity and high temperature
conditions can be used for medical instruments and microscopes for in vivo operations.
1.2.3
Current Research Advancements
The current research attempts to understand what effect adding counter-ions as an
additional modification step to the pre-existing PEM will have on hydrophilicity and
wetting. This was done by modifying the pre-existing PEM structure with monovalent
and divalent cations and observing the change in antifogging properties. If the addition of
ions aids the superwetting and wicking characteristics of these films, it may greatly
impact the design of future PEMs. The cations interact with the carboxylic acid groups
from the PAA, as shown in Figure 4. The cations may provide some extra hydrophilic
characteristics, and may improve antifogging properties after humidity aging.
Furthermore, the divalent cations may provide a source of internal cross-linking and
enhance the mechanical stability of these films.
NaCOO
,
_
Na'COO0-
Addition of Monovalent Counter-ion
COO
Addition of Divalent Counter-ion
Figure 4. Introduction of cations to APSiO 2/PAA film
Chapter 2: Design Parameters
2.1 Introduction to Design Idea
This project proposes to increase anti-fogging behavior in PEM films by the addition of
cations. The PEM studied is a multilayer of APSiO 2 nanoparticles and PAA. Under
various synthesis pHs of these films, the PAA films will contain different amounts of free
carboxylic acid groups. When these PEMs are dipped in salt solutions, the free carboxylic
acid groups will interact with the counter-ions. The adsorbed counter-ions will give the
PEM a new antifogging characteristic depending on the new hydrophilicity of the film. In
addition, divalent cations may interact with multiple carboxylic acid groups, promoting
cross-linking and a higher mechanical stability and durability. However, prior to dipping
PEMs in counter-ion solutions, it was important to find a stable PEM system to use for
this experiment.
2.2 Exploratory Experimentation
2.2.1
Thickness Consideration and Calibration
The thickness of the grown films should be consistent in order to ensure proper
comparisons across different systems. A thickness of 100 nm was chosen as a suitable
thickness to preserve optical transmittance and interference color differentiation. At
around 100 nm, the PEM film takes on a purple hue. However, the interference color
behaves in a sinusoidal manner with relation to thickness, and it is easy to observe
perturbations in thickness by the interference color observed in the film.
2.2.2 Free Carboxylic acidic acid Group Content
Counter-ions will adsorb to the film at sites in which there are free carboxylic acid groups
available in the PAA layers. Experimentally, it is important for a large amount of free
carboxylic acidic acid groups to be present in the films so counter-ions may interact with
them in order to clearly observe the effect the counter-ions have on antifogging
properties. Although all carboxylic acidic acid groups may not interact with counter-ions,
it was important to choose a PEM system that had the most free carboxylic acidic acid
groups in order to attract the largest number of counter-ions.
In order to determine the free carboxylic acidic acid group content of the films, they were
stained with methylene blue (Figure 5). Methylene blue is a blue dye which binds to
negative charges, and based on the density of the dye, will help determine the relative
amount of free carboxylic acid groups. As the films are dipped in methylene blue
solution, the positively charged nitrogen in the methylene blue interacts with free
carboxylic acid groups and the dye molecule is adsorbed to the film.
CH3
I
SIN
HaC
CH3
I
N
C
CH 3
Figure 5. Methylene blue molecular structure.
2.2.3
Film Stability Under pH Stress
It was important to observe the stability of the PEM system under various pH stresses
before carrying out the counter-ion experiments. Films that degrade under pH conditions
other than that in which they were made are not as suitable for the counter-ion test,
because it is important to introduce the counter-ions at a neutral pH in which they would
bind to the free carboxylic acid groups more abundantly.
2.3 Exploratory Experimental Methods
2.3.1
PEM Films on Glass Slides
Initially, PEMs of APSiO 2/PAA were grown on clean glass slides (VWR microslides,
premium, plain from VWR). The slides were cleaned by sonication in solutions of soapwater, 1M NaOH, and milliQ DI water for 15 minutes, 15 minutes, and 5 minutes
respectively. In order to test different systems of PEMs, they were grown with different
pH solutions of APSiO 2 and PAA. The systems tested were at pH 3.0, pH 2.5, and pH
2.0. Solutions of APSiO 2 were made by dissolving 4.0 g APSiO 2 in 500 mL milliQ DI
water and titrating to the correct pH with HCI (Hydrochloric Acid solution, 1.0 N from
Sigma). Solutions of PAA were made by dissolving 0.14 g PAA (MW -90,000, 25%
aqueous solution from Polysciences Inc.) in 500 mL miiliQ DI water and titrating to the
correct pH with HC1. Using a StratoSequence VI (nanoStrata Inc.) spin dipper and the
StratoSmart v6.2 (nanoStrata Inc.) software, a bilayer film was grown by first dipping
slides in the APSiO2 solution for 10 minutes, followed by dips in three water rinses of
equal pH as the solutions for 2 minutes, 1 minute, and 1 minute. This was followed by
dipping slides in the PAA solution for 10 minutes, followed by dips in three water rinses
of equal pH as the solutions for 2 minutes, 1 minute, and iminute. To create more
bilayers, this process was repeated. To find the number of bilayers in each system to
reach the appropriate thickness of 100 nm, calibration curves were constructed. Film
thickness was measured using the P10 Profilometer (Tencor).
2.3.2
Film Stability Under pH Stress
Due to the growth of these films under different pHs, the stability of each film was tested
at various pHs. To identify disparities between the systems, a film of each was immersed
in water at pH 9.0, pH 6.0, pH 4.0 and their growth pH (3.0, 2.5, or 2.0) and the resulting
film was observed by visual inspection for alterations in slide coverage and by
profilometer thickness measurements.
2.3.3
Free Carboxylic acid Group Content
Free carboxylic acid group content was measured by method of methylene blue staining.
Methylene blue solutions were made by dissolving 0.015 g crystalline Methylene Blue
(Alfa Aesar) in 40ml yellow pH 7 buffer (BDH). Films were stained by immersing the
slides in this solution for 15 minutes, followed by immersion in three rinses of milliQ DI
water for 2 minutes, 1 minute, and 1 minute. One untreated PEM film of each pH system
was stained with methylene blue, as were all films that were pH treated. The stained films
were then analyzed using a Cary 500i Spectrophotometer (Varian, Inc.) to measure the
relative absorbance around the blue wavelength.
2.4 Exploratory Experimental Results
Calibration experiments showed optimal number of bilayers on glass slides to be eight
bilayers for a pH 3.0 system, seven bilayers for a pH 2.5 system, and six bilayers for a pH
2.0 system. Films did not show a linear response to additional bilayers, as can be seen in
Figure 6. Unpublished results from the Rubner Group have also found that film growth
for PEMs follow an exponential growth curve. Because the target film thickness was
100nm, additional points were not added to the calibration graph once this goal was
reached.
pH 3.0 Film Thickness Calibration
200
150
100
50
0
8
6
4
2
# Bi-Laye rs
pH 2.5 Film Thickness Calibration
V3/
150
100
50
2
6
4
# Bi-layers
pH 2.0 Film Thickness Calibration
200
150
100
50
0
2
4
6
8
# Bi-layers
Figure 6. Calibration curves for films grown at a) pH 3.0, b) pH 2.5, and c) pH 2.0 with
2 nd degree polynomial fitted trendline.
From the post assembly pH treatment studies on each of these systems, some delamination or degradation was observed for all films at pH 9.0. However, the films made
at pH 3.0 showed very slight delamination around the slide, while the films made at the
other two pHs showed marked film degradation. Table I lists the observations of the
films upon removal from the pH dip. Figure 7 shows a series of pictures comparing
different degrees of degradation in these films. These films were methylene blue stained
in order to characterize their free carboxylic acid content, explaining the blue/purple
tinge.
Table 1. Results of pH dips for different growth systems.
Growth pH
3.0
3.0
3.0
3.0
2.5
2.5
Post assembly treatment pH
3.0
4.0
6.0
9.0
2.5
4.0
Result
No Change
No Change
No Change
Slight Delamination
No Change
No Change
6.0
9.0
2.0
4.0
6.0
9.0
2.5
2.5
2.0
2.0
2.0
2.0
No Change
Considerable Degradation
No Change
No Change
Slight Degradation
Considerable Degradation
Figure 7. Degradation of pH treated films: a) growth in pH 3.0, treated with pH 6.0, b)
growth in pH 3.0, treated with pH 9.0, c) growth in pH 2.5, treated with pH 9.0, and d)
growth in pH 2.0, treated with pH 9.0.
Results of the methylene blue treatments and spectrophotometer measurements, shown in
Table 2, show similar results for all films treated in growth (2.0-3.0) pH, pH 4.0, and pH
6.0. Each of these had an absorbance between 6.5 and 6.9 times the absorbance of an
unstained film at 500 nm. While the difference between 6.5 and 6.9 times the absorbance
may seem large, in fact small alterations in the placement of the film could result in
absorbance changes of up to 0.6 times the absorbance of an unstained film.
Table 2. Methylene blue stained absorbance results of post-assembly treated films.
Growth pH
3.0
3.0
3.0
3.0
3.0
2.5
2.5
Post assembly treatment
pH
none
3.0
4.0
6.0
9.0
none
2.5
Relative
Absorbance
6.8
6.7
6.7
6.6
6.9
6.9
6.9
2.5
2.5
2.5
2.0
2.0
2.0
2.0
2.0
4.0
6.0
9.0
2.0
3.0
4.0
6.0
9.0
6.8
6.7
6.3
6.6
6.7
6.7
6.5
5.8
2.5 Experimental Setup Conclusions
Free carboxylic acid group content was not significantly different for all systems after
treatment in pH 6.0, and de-lamination and degradation occurred in all systems for dips in
pH 9.0. However, the amount of degradation was least in films made at pH 3.0, and this
system was determined to be the best to observe the counter-ion influence on antifogging
PEMs.
Chapter 3: Counter-ion Influence on Antifogging PEMs
3.1 Polyelectrolyte Films on Polycarbonate Slides
PEMs of APSiO2/PAA were grown on plasma cleaned polycarbonate slides. The slides
were plasma cleaned by engulfing the slides in oxygen gas at 400 mTorr and creating an
ionizing electric field for ten seconds. Films were grown using the same method as on
glass slides, and a calibration curve was constructed to find the correct number of
bilayers to get the appropriate 100nm thickness. Polycarbonate was chosen as a substrate
because of the demand for its use in commercial applications. Applications that
specifically would benefit from antifogging coatings include eyeglasses and sunglasses,
car headlights, and various medical devices.
3.2 Counter-ion Substitution
0.1M solutions of NaCl (from Sodium Hydroxide, 1.000 - VWR), LiC1 (from Lithium
Hydroxide, Reagent grade, >98%, powder from Sigma Aldrich), and KCl (from
Potassium Hydroxide, Pellets, 99.99% metals basis, semiconductor grade from Sigma
Aldrich), and 0.05M solutions of MgC12 (from Magnesium Hydroxide, Ultra >99% from
Fluka), CaCI2 (Calcium Hydroxide puriss. from Riedel-deHadn), and BaC12 (Barium
Hydroxide, tech., -95% from Aldrich) were made and titrated to a pH of 7. The PEM
films dipped on polycarbonate slides at pH 3.0 were immersed in these salt solutions for
an hour, after which they were immersed in a milli-Q DI water rinse for 15 minutes. At
the pH the dip was conducted at, and, later, at ambient conditions, it appeared that the
magnesium particles precipitated and is the cause for the cloudy nature of the film. Even
under room temperature and humidity, the Mg 2+ film was noticeably cloudy, so the effect
of introducing magnesium counter-ions to a PEM film must be deemed inconclusive, and
the results were removed from the results section. In order to prevent precipitate from
forming, the salt solutions may be brought to a lower pH. This would decrease the
amount of magnesium salt precipitate formed.
3.3 Film Characterization
3.3.1
Anti-Fog Test
Anti-fog tests were performed on all the films to ascertain their anti-fogging
characteristics. This test consisted of introducing the PEM film to high humidity and high
temperature conditions (80% humidity and 370C) and taking photographs after 0, 10, and
20 seconds using a Fujifilm camera.
3.3.2
Contact Angle Measurement
Contact angle measurements were taken with a VCA2000 Video Contact Angle System
(AST Inc.) goniometer using the dynamic mode of the VCA OptimaXE (AST Inc.)
software. The contact angles were measured by dropping a single droplet of water onto
the PEM film and measuring the angle the droplet made with the surface. Contact angles
were measured at the time the droplet hit the surface and at 0.1 second intervals up to half
a second, as well as the contact angle after approximately three seconds.
3.3.3
Durability Test
A mechanical durability test was applied to the films by rubbing them with a
KimtechScience Kimwipe (Kimberly-Clark Professional). They were each rubbed gently
by hand and then more forcibly until an inference of relative stability could be made
about the film.
3.3.4 Humidity Chamber Aging
After these tests were performed, the films were aged in the high humidity and high
temperature chamber (80% humidity and 370C) for three days. After each day, anti-fog
tests and contact angle measurements were repeated.
3.3.5 Ellipsometry Measurements
Measurements were taken of each film with a XLS-100 (J.A. Woollam Co., Inc.)
spectroscopic ellipsometer to ascertain the thickness and refractive index. The
ellopsometer was powered by a M-2000D (J.A. Woollam Co., Inc.) power source, and
ran with the Wvase32 (J.A. Woollam Co., Inc.) software package. The scans were taken
at a 70 degree angle and from a wavelength of 250nm to 40000nm.
3.4 Results
Contact angle measurements were taken from pre-aged films until the third day of aging
in the humidity chamber. Figure 8 shows the contact angles of a drop of water on the film
from the moment a drop touched the film surface until half a second afterwards. The four
graphs represent pre-aging through three days of humidity aging, and each contain data
from the six counter-ions tested as well as an unaltered 10 bilayer APSiO 2/PAA film.
Before the films are placed in the humidity chamber, day zero, all modified films show
superior hydrophilicity to the unaltered film, as shown by their smaller contact angles.
After one day in the humidity chamber, the films modified by monovalent cations show
superior hydrophilicity to the unaltered film, while the divalent cations exhibit less
hydrophilic character than the unaltered film. By day two in the humidity chamber, the
trend seen in day one is seen more pronouncedly: the films modified by monovalent
cations exhibit an inverse relation between size and hydrophilicity, and continue to show
significantly higher hydrophilic character than the unaltered film. Also, after two days in
the humidity chamber, the films modified by the smaller divalent cations, Mg2+ and Ca 2+ ,
have comparable or slightly lower contact angles than the unaltered film. After three days
in the humidity chamber, the monovalent cation-modified films show comparable contact
angle measurements to each other half a second after the water droplet makes contact
with the film, and are still significantly more hydrophilic than the unaltered film.
Similarly, the smaller divalent cations continue to have comparable contact angles to the
unaltered film, while the Ba 2+ -modified film continues to exhibit a lesser degree of
hydrophilicity.
]
Day Zero: Contact Angle vs. Time
60
50
40
d
30
20
10
CLi2+
Ba2+
K+.
Na+
I
I
I
0.2
0.1
0.3
0.4
0.5
Time (sec)
Day One: Contact Angle vs. Time
60
" 50
40
Q 30
2+
_C
S20
~
0
--
Unaltered 1
0
-'9--I
0.1
0.2
0.3
Time (sec)
-.
0.4
---------
Li +
K
0.5
L
Day Two: Contact Angle vs. Time
70
60
--
50
40 -
0
0.1
0.2
0.3
Time (sec)
0.4
0.5
Day Three: Contact Angle vs. Time
60
I
40
200
0 10
-
Na+
U-naltered
-___
0
0.1
0.2
0.3
0.4
0.5
Time (sec)
Figure 8.Dynamic contact angles of counter-ion dipped films from 0-0.5 seconds after
water droplet contact after a)no aging, b)one day aging, c)two day aging, and d)three
day aging inhumidity chamber.
Figure 9shows the contact angles on the seven films after the water droplet had been in
contact with the film for approximately three seconds as a function of humidity aging
time.
The films modified by monovalent cations have significantly lower contact angles
time. The films modified by monovalent cations have significantly lower contact angles
than the unaltered films despite aging. As observed earlier, the larger monovalent ions
seem to correlate with increased hydrophilicity and lower contact angle. Furthermore, the
monovalent cation-modified films do not have a large change in contact angle under the
humidity aging. On the other hand, films containing divalent cations showed noticeably
increased contact angles after due to aging.
Contact Angles vs. Aging Time
I 1r
4'
S40
• 35
Ca2 +
Unaltered
30
225
20
15
Na+
LiK+
5
n
I
SI
0
1
2
Aging Time (days)
3
Figure 9. Final contact angles of counter-ion dipped films from zero to three days of
aging in humidity chamber.
Table 3 shows the results of the anti-fog tests before, during, and after humidity aging for
three days. The anti-fog tests show very good anti-fog properties for all films. However,
after aging, the divalent cation-modified films lose their anti-fog property more quickly
than the unaltered film. While the Na+-modified film performed worse after 2 days of
aging than the unaltered film, both the Li +- and K+-modified films performed better or on
par after both one and two days of aging. Table 3 also shows evidence to counter the
argument of antifogging dependency on contact angle (shows in parentheses in Figure 2).
Typically it has been observed that with <5 degree water droplet contact angles, films
show superb antifogging characteristic.1,2 However, the data here suggests that
antifogging properties may be more dependent on some other feature of a film, as there is
not strong correlation between antifogging properties and the contact angle. Figure 10
shows a comparison of the unaltered film, the Na+-modified film, and the Li+-modified
film after two days of humidity aging. We see that Li+-modified films may have
outperformed the unaltered film in the anti-fog test over time.
Table 3. Development of anti-fogging properties over humidity aging time (contact
angles in degrees in parentheses)
Film
Unaltered
Na
Li
K
Ca
Ba
Figure 10. Anti-fog test of a) unaltered, b) Na -modified, and c) Li'-modified films after
two days of aging in the humidity chamber.
The mechanical durability was tested with a Kimwipe rub test. Table 4 shows the
durability on a scale of 1 to 10, with a rating of 1 meaning the film was virtually
impossible to rub off with the load of a human finger enclosed by a Kimwipe fully
pressed against the film, and a rating of 10 meaning the film easily flaked off. Because all
of the films were very durable, a rating of 2 or 3 meant that the film rubbed off under
medium load from the finger. Most of these films did not damage or damage greatly
under the rub test, and all of them showed good strength. There also seems to be no trend
associated to durability that can be attributed to any specific aspect of these cations.
Table 4. Mechanical durability of counter-ion films under rubbing test
Film
Observation
Numerical Rating
Unaltered
Extremely Durable
1
Na
Very Durable
2
Li
Extremely Durable
1
K
Very Durable
2
Ca
Durable
3
Ba
Extremely Durable
1
Table 5 shows the thicknesses and refractive indices of the films from ellipsometry data
before and after humidity aging. Although no significant trends can be observed in the
pre-aged and post-aged thicknesses, there is a trend to be noticed in the refractive indices
of the films. It appears that the films with monovalent cations have lower refractive
indices, while the divalent cation containing films have higher refractive indices. We can
hypothesize that the films with lower refractive index have increased porosity, and the air
(n = 1.00) permeating these pores is bringing the overall refractive index of the film
down. Similarly, comparing the divalent modified films with the unaltered films leads us
to suggest that the higher refractive index of the divalent modified films shows lower
porosity and may indicate cross-linking may have occurred, encouraging a denser film
with lower porosity. Table 6 shows the same ellipsometry data, but without a modeled
roughness layer. This roughness layer is put in to account for film porosity that affects the
refractive index. The roughness layer is modeled to be half air, half film material. The
similarity between the two shows that there is likely at least a semi-uniform structure
throughout the layers of the film. If this were not the case, the refractive indices would be
largely different between the two tables.
Table 5. Film thickness and refractive index pre- and post-humidity aging with an
included modeled semi-porous roughness layer
Aged
Unaged
Unaltered
Roughness
Layer
Thickness
2 nm
Unaged
Film Total
Thickness
114 nm
Roughness
Layer
Thickness
5 nm
Aged Film
Total
Thickness
120 nm
Unaged
Refractive
Index
1.43
Aged
Refractive
Index
1.47
Na
Li
0 nm
0 nm
115 nm
129 nm
10 nm
11 nm
112 nm
115 nm
1.35
1.34
1.41
1.42
K
23 nm
122 nm
40 nm
127 nm
1.35
1.39
Ca
Ba
6 nm
8 nm
114 nm
117 nm
0 nm
2 nm
115 nm
129 nm
1.47
1.50
1.48
1.49
Table 6. Film thickness and refractive index pre- and post-humidity aging without
modeled semi-porous roughness layer
Unaltered
Na
Li
K
Ca
Ba
Un-aged
3 Day Aged
Un-aged
Refractive
3 Day Aged
Refractive
Thickness
Thickness
Index
Index
1.43
1.35
1.34
1.34
1.47
1.49
1.46
1.40
1.41
1.41
1.48
1.49
114 nm
116 nm
130 nm
130 nm
113 nm
112 nm
120
114
116
116
118
129
nm
nm
nm
nm
nm
nm
3.5 Discussion
Antifogging properties largely seem to correlate with low contact angle measurements.
This supports the findings of Cebeci and coworkers and corroborates the idea of
nanopores wicking moisture into the film to create robust antifogging properties.'
An interesting feature in this experiment is the difference between the effects of
monovalent and divalent cations on the PEM film characteristics. Monovalent cations
caused lower refractive indices which we attribute to supposed increased porosity. The
foundation of the creation of this porosity is unclear, and literature supports two ideas.
The first possibility is that the salt solution causes a rearrangement of the film. 4 When the
salt solution is added, the ions may cause the PAA to coil around the nanoparticle, and
allow the salts into the film. This rearrangement process may cause an increase in
nanoporosity. Another theory is that the salt solution shields the charged electrolytes and
cause a mass loss as PAA is removed from the film into solution.5' 6 In this scenario,
nanoporosity would similarly be increased altering the packing density of the remaining
film altering the refractive indices. From current data, it would be hard to choose between
these two possibilities, and it is likely that both mechanisms are taking place in these
films upon salt treatment. The divalent cations, on the other hand, caused higher
refractive indices than in the untreated film which could be directly attributable to a lower
porosity due to the higher density caused by ionic crosslinking. However, an explanation
of why neither mass loss nor film rearrangement play a large role cannot be given with
the current data. An AFM measurement of porosity would be very useful in clearing this
up.
Antifogging properties are tied into the porosity of the film, as the porosity has a large
factor in capillary condensation. The benefits from increased porosity can be seen in
some of the antifogging tests conducted on the monovalent cation modified films, as they
generally performed as well or better than the unaltered films.
Contact angles can, for the most part, be explained by changes in porosity as well. As the
pores filled more quickly due to capillary condensation in the less porous films, their
contact angles increased rapidly. Conversely, the more porous films took more time for
their antifogging properties to degrade and their contact angles to increase. However, as
aging time increases, all films inevitably lose their antifogging properties.
An outlier to the contact angle trend with porosity is the set of pre-aging contact angle
data. Before aging, the treated films, including the divalent films, all had lower contact
angles than the untreated film. In this case, is likely that the increased hydrophilicity of
the film due to cation addition was still able to direct water molecules into the pores of
the films. After aging, the nanopores may have become saturated, explaining the large
change in contact angles of the divalent films.
Chapter 4: Summary and Future Work
4.1 Thesis Summary
The effects of counter-ions on antifogging properties of PEMs was investigated in this
thesis. Interesting differences were found between monovalent and divalent cationic
effects on the films. Monovalent cations may prove to be useful in increasing antifogging
durability. More importantly, from this study, it is clear that salt solutions have a definite
impact on PEM films and their water droplet contact angles and antifogging properties
and warrant further investigation.
4.2 Future Directions
The outcome of this work has posed some interesting questions about why these counterions act the way they do. A study to determine the nano-porosity and topography of
counter-ion treated PEM films using an atomic force microscope (AFM) may shed light
on the film characteristic differences observed between the monovalent and divalent
counter-ions. Furthermore, it may show why although the contact angles for the
monovalent counter-ion treated films was consistently low with humidity aging, and
indeed much lower than the untreated film, the anti-fogging properties were not as
effected.
More work should also be done in pH control when adding these counter-ions.
Particularly, magnesium has a low solubility in water, and may be more likely to dissolve
and consequently interact with PEMs at lower pHs. The pH at which all of these counterions are added may make a significant impact in the adsorption ratio, and therefore a
significant impact on the hydrophilicity and anti-fogging properties.
Another method of treating PEMs that could be tried with counter-ions is that of
calcination. Heat treating these counter-ions may impact them in a beneficial way.
There are a variety of experiments that can arise from the findings of this thesis, and
hopefully some of these will be undertaken in the search for durable anti-fogging films.
4.3 Bibliography
(1) F. C. Cebeci, Z. Wu, L. Zhai, R. E. Cohen, M. F. Rubner, Langmuir 2006, 22, 2856
(2) J. A. Howarter, J. P. Youngblood, DOI: 10.1002/marc.200700733
(3) R. N. Wenzel, Ind. Eng. Chem. 1936. 28, 988.
(4) A. Fery, B. Scholer. T. Cassagneau, F. Caruso, Langmuir 2001, 17, 3779
(5) S. T. Dubas, J. B. Schlenoff, Macromolecules2001, 34, 3736
(6) D. Kovacevic, S. van der Burgh, A. de Keizer, M. A. Cohen Stuart, Langmuir 2002,
18, 5607
A
PPiNnDTY
A
~I
1 ul~
Da.Y-.,Zer
,DiByTo
Onme
I-Da Thee
Figure 11. Anti-tog tests on unaltered tilms betore and during humidity aging.
Day One
mDyT
_I
!T'hei e
Figure 12. Anti-tog tests on Na-moclilieca tllms tetore ana cauring numlalty aging.
,Pyi
Day Two
--
I~s~a~lP
IDa
he
Figure 13. Anti-ftog tests on Li-moditied tilms betore and during thumallty aging.
,-*C ,;k
"
Da' Tw
~o~enai
iDd"
hree
Figure 14. Anti-fog tests on K-modified films before and during humidity aging.
4 '. '
-tl'
.4-7777
DaylEne
DayThre'
__
Figure 15. Anti-tog tests on Mg-mocitled tllms betore an
°__
auring numlaity aging.
'DaNTa
Day,,One:ý
'Da"'""'Tw
Day htte`ý,'
Figure 16. Anti-fog tests on Ca-modified films before and during humidity aging.
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